1. Field of the Invention
This invention relates to methods of treating aqueous salt solutions to provide a solution suitable for vitrification to a stable glass matrix for long term storage. The present process involves removal by crystallization of burkeite, a congruent double salt with the chemical formula, Na6(SO4)2(CO3), and sodium complex salts from aqueous salt solutions. In particular, salt solutions composed of aqueous nuclear waste materials are suitable for treatment by the invention. Specifically, salt solutions which have a sulfate to sodium mole ratio that does not permit easy vitrification into stable glasses may be treated by the present invention.
2. Discussion of the Related Art
Large volumes of radioactive aqueous waste have been generated during plutonium production and other nuclear operations. These wastes are stored in storage tanks at various locations, for instance, the U.S. Department of Energy""s (xe2x80x9cDOExe2x80x9d) Hanford, Washington site. At present, the DOE Hanford site stores approximately 50 million gallons of radioactive aqueous waste. Major components in the waste include various soluble and insoluble compounds including salts of sodium, aluminum, and phosphorous including some of the sulfate, nitrate, nitrite, oxide, carbonate, and hydroxide salts of those metals. In these complex mixtures, the solubility of a specific salt will obviously depend on such factors as temperature and pH of the mixture. Radioactive components in the aqueous waste include strontium, cesium, technetium, cobalt, uranium, and plutonium.
Present plans for disposal of these waste solutions call for the vitrification of the liquid wastes into glass matrices suitable for stable long-term storage. The vitrification process requires that the composition of the waste solution fall within certain parameters to ensure production of a stable glass matrix without formation of undesirable salt phases. One such parameter is the sulfate to sodium mole ratio, which if in excess of 0.01 SO4xe2x88x922/Na+, the sodium sulfate present may exceed the glass solubility limit, and form a non-miscible salt phase during the vitrification process. These non-miscible sulfate salts can corrode the refractory lining of ceramic-lined vitrification units, and thus significantly reduce the operating life of the vitrification equipment. Furthermore, in sufficient quantity, the accumulation of non-miscible sulfate salts provides an electrically preferred circuit in Joule-heated vitrification melters, thus bypassing the molten glass matrix and significantly reducing the waste processing rate. Presently, the sulfate to sodium mole ratio in numerous DOE Hanford aqueous waste tanks exceeds the glass solubility limit.
Sodium sulfate can be a major contaminant to the vitrification process. At DOE Hanford, for instance, sodium sulfate accounts for approximately 3.3% of the total sodium salts. Under the current processing approach to meet the sulfate vitrification specification, additives, such as sodium hydroxide, must be added to the waste to reduce sodium sulfate to less than 2% of the total sodium salts. By this method, the amount of vitrification glass and processing time are increased proportionally to the amount of sodium hydroxide added to the waste.
Sodium sulfate has limited solubility in glass. In equilibrated solutions, sodium sulfate solubility in glass is 0.5 wt % SO3 at a Na2O concentration of 14 wt. %. See VSL-00R3630-1, Summary of Preliminary Results on Enhanced Sulfate Incorporation During Vitrification of LAW Feeds, I. L. Pegg, et al., Vitreous State Laboratory, The Catholic University of America, Washington D.C. This corresponds to a SO4xe2x88x922/Na+ mole ratio of 0.013. Experimental testing of glass mixtures indicates that a molten sulfate salt layer forms well before the glass melt is saturated with sodium sulfate. The sulfate concentration at which a salt phase forms has not been quantified, but thick salt layers have been observed in test melters at less than three-quarters of sulfate glass saturation.
Presence of a molten sulfate salt phase in the melter is highly undesirable for several reasons. Feeding slurry onto a molten sulfate layer could cause over-pressurization or steam explosion in the melter. See DOE/RL-98-01, Rev. 3 Sulfate Mitigation for Hanford Tank Low Activity Waste Vitrification, Technology Needs/Opportunities Statement RL-WT101. Molten sulfate salts are more corrosive than the glass melt, can penetrate the refractory of the melter, and cause electrical shorting and corrosion of the melter components. These molten sulfate phases also tend to sequester a variety of hazardous and radioactive elements, including, for example, cesium and chromium. Furthermore, the sulfate salts are highly soluble in water, which renders the glass product unacceptable for long-term storage.
In addition, sulfate may be reduced to sulfur dioxide in the melter, which may be absorbed in a caustic scrubber, to be recycled to the melter, or it may escape the system altogether and become an atmospheric pollutant.
Clearly, the presence of sulfate in the vitrification system is detrimental to safety, the equipment, the resulting glass, and the environment. At the current sulfate feed concentrations of DOE Hanford aqueous wastes, sodium sulfate will frequently be above the saturation concentration in the glass and thus a molten salt phase is expected to occur. The DOE Hanford aqueous waste contains approximately 4,800 metric tons of sodium sulfate.
The current processing approach to adjust the sulfate/sodium ratio to below 0.010 SO4xe2x88x922Na+ is to add additives, such as sodium hydroxide, to the aqueous waste until the desired ratio is reached. By this method, the resulting amount of glass and processing time, are both increased by approximately 86% over a non-sulfate containing vitrification feedstock. Additionally, the addition of the required amounts of sodium hydroxide significantly increases the treatment cost.
Others have tried unsuccessfully to adjust the sulfate/sodium ratio by decreasing the sulfate concentration through removal of sulfate by evaporation and selective precipitation of sulfate, and concluded that evaporation is not a viable option for removing sulfate. See PNWD-3036; BNFL-RPT-018, Removal of Sulfate Ion From AN-107 by Evaporation, G. J. Lumetta, et al., Pacific Northwest National Lab., Richland, Wash.
Clearly there is a need for a method to process radioactive aqueous waste with high sulfate/sodium ratios generated by nuclear activities to permit such waste to be vitrified into stable glass matrices. Ideally such a sulfate/sodium ratio adjustment process would decrease chemical costs, processing expense, and volume of glass matrix produced, while removing both sulfate and sodium containing compounds in a stable form.
The present invention meets the above-stated needs and overcomes the drawbacks of the current processing approach by removing sulfate, in the form of stable burkeite, to adjust the sulfate/sodium ratio of aqueous radioactive waste solutions to permit vitrification without increasing glass volume with additives or forming non-miscible sulfate salts. In particular, the present invention accomplishes these objectives by providing a process for the removal of burkeite from a salt solution containing aluminum ions, nitrite ions, nitrate ions, sodium ions, calcium ions, and sulfate ions. The inventive process involves either adding water and heating, or adding alkali to form a solution containing alkali soluble compounds, followed by optionally filtering the solution to remove any undissolved solids and to produce a solution essentially free of solids. The undissolved solids are typically wastes which are higher in radioactivity than the solution. Excess water is then evaporated from the solution essentially free of solids to produce a saturated solution from which burkeite is precipitated, by evaporative crystallization, and separated.
The present inventive process may reduce sulfate by an order of magnitude, well below the glass solubility limit, and prevent a molten salt phase from forming in the melter.
The present inventive process for the removal of burkeite from a salt solution containing aluminum ions, sodium ions, calcium ions, and sulfate ions involves adding alkali to a salt solution comprising salts containing aluminum ions, nitrite ions, nitrate ions, carbonate ions, sodium ions, calcium ions, and sulfate ions to form a solution containing alkali soluble compounds, then optionally filtering the solution to remove any undissolved solids and to produce a solution essentially free of solids. Excess water is then evaporated from this solution to produce a saturated solution. Burkeite is then precipitated, preferably by evaporative crystallization, from the saturated solution, and finally separated from the saturated solution.
Additional processing measures which may also be applied to the salt solution, prior to the evaporating step, include oxidizing any organic compounds present in the salt solution, precipitating any strontium containing compounds present in the salt solution, and removing any radionuclides present in the salt solution. Among the radionuclides which may be removed are cesium and technetium, typically ion exchange may be used.
The salt solutions processed by the present inventive method may be composed of salts which contain at least one negatively charged member selected from the group consisting of, for example, carbonate, chromate, fluoride, hydroxide, nitrite, nitrate, oxide, silicate, sulfate, and phosphate, and one positively charged member selected from the group consisting of, for instance, aluminum, barium, calcium, cesium, iron, manganese, nickel, sodium, strontium, technetium, plutonium, potassium, uranium, and zirconium.
After the burkeite is separated from the saturated solution, it may be solubilized and reprecipitated by addition of either or both of calcium carbonate and barium sulfate, using conventional technology.
Typically alkali may be added only until any amphoteric aluminum compounds present in the salt solution are solubilized. Alkali compounds that may be used for dissolving aluminum solids include, for example, sodium hydroxide, potassium hydroxide, calcium hydroxide, and other hydroxide containing compounds.
Evaporative crystallization may be utilized to precipitate burkeite from the saturated solution. This evaporative crystallization may occur within a forced circulation evaporator. It is preferable that the saturated solution be essentially free of solids prior to undergoing evaporative crystallization.
After separation of the precipitated burkeite the saturated solution may be vitrified according to methods known in the art. The saturated solution may be vitrified alone, or it may be combined with other salt solutions then vitrified. In some cases, additional Nat may need to be added to reach a suitable SO42/Na+ mole ratio to produce a glass matrix suitable for long-term storage.
Another embodiment of the present invention is a process for the removal of burkeite from a radioactive salt solution by treating the radioactive salt solution to produce a saturated salt solution essentially free of solids. Burkeite is then precipitated from the saturated salt solution, and finally separated from the saturated salt solution.
Additional processing steps that may occur prior to precipitating burkeite include adding alkali, separating alkali insoluble compounds, oxidizing organic compounds, precipitating strontium containing compounds, removing radionuclides, and then evaporating excess water.
Yet another embodiment of the invention is a method to reduce the sulfate concentration of a salt solution by adding alkali to a salt solution comprising salts containing aluminum ions, nitrite ions, nitrate ions, carbonate ions, sodium ions, calcium ions, and sulfate ions to form a solution containing alkali soluble compounds. Optionally, this solution may then be filtered to remove any undissolved solids and to produce a solution essentially free of solids. The undissolved solids are typically higher in radioactivity than the solution. Excess water may be evaporated from the solution essentially free of solids to produce a saturated solution. Sulfate-rich material may be precipitated from the saturated solution, and then the precipitated sulfate-rich material may be separated from the saturated solution.
A different embodiment of the present invention may be a process for the removal of burkeite and sodium-containing complex salts from a salt solution containing aluminum ions, nitrite ions, nitrate ions, carbonate ions, sodium ions, calcium ions, fluoride ions and sulfate ions by adding hydroxide to a salt solution comprising salts containing aluminum ions, nitrite ions, nitrate ions, carbonate ions, sodium ions, calcium ions, fluoride ions, and sulfate ions to solubilize aluminum containing compounds thereby to form a solution containing soluble aluminum compounds from the salt solution. Optionally, any undissolved solid material may be filtered off from the solution containing soluble aluminum compounds to produce a solution essentially free of solids. Excess water may be evaporated from the solution essentially free of solids to produce a saturated solution. Burkeite and sodium-containing complex salts may be precipitated from the saturated solution, and then the precipitated burkeite and sodium-containing complex salts may be separated from the saturated solution.
This embodiment may further include: one or more of the following steps as known in the art, prior to the evaporating step: oxidizing any organic compounds present in the salt solution, precipitating any strontium containing compounds present in the salt solution, and removing any radionuclides present in the salt solution.
The radionuclides may be cesium and technetium, and may be removed by ion exchange, or other conventional ion separation techniques.
The saturated solution may be vitrified after separation of the precipitated burkeite and sodium-containing salts to obtain a glass matrix suitable for long-term storage. The vitrification may be accomplished by conventional processes.
A further embodiment of the present invention involves the co-crystallization and removal of sodium-containing complex salts, such as, the carbonate, sulfate, and fluoride salts of sodium along with the burkeite. Additional sodium carbonate salts include, for example, the anhydrate, and the mono-, hepta-, and deca-hydrates. Further sodium sulfate compounds include sesquiburkeite (Na2SO4.Na2CO3) and anhydrous sodium sulfate. Sodium fluoride salts may include sodium fluoride, and trisodium fluoride sulfate.
These sodium-containing complex salts may be crystallized and removed during the burkeite evaporative crystallization process. The removal of these sodium salts has the additional benefits of further reducing the volume of radioactive glass, avoiding increased corrosive effects, and increasing glass stability.
Additional features, advantages, and embodiments of the invention may be set forth or apparent from consideration of the following detailed description, drawings, and claims. Moreover, it is to be understood that both the foregoing summary of the invention and the following detailed description are exemplary and intended to provide further explanation without limiting the scope of the invention as claimed.